For me, I enjoy onsen outdoors. The cool air above at your head while the warm water attempts to slowly blanch you.

Indoor onsen does not make much sense. You need to look out to see the sky and the occasional overhanging branches blocking partially your view of the sun and clouds.

Also for onsen to really work, every user really needs to take care of their personal cleanliness and hygiene. That is why I stopped using public Jacuzzis at cruises. Its like a melting pot of flavours, fats and oil.